Conjugation in Alkadienes and Allylic Systems

1. Conjugation in Alkadienes and Allylic Systems

2. A double bond can act like a substituent and give other groups special properties and reactivity.For example carbocations, radicals and anions connected to alkenes are called allylic carbocations, allylic radicals and allylic anions. Conjugation in Alkadienes and Allylic Systems Alkenes connected by a single bond are called conjugated dienes.

3. The Allylic Group Allyl is both a common name and a permissible IUPAC name for the H2C=CHCH2 group.The sp3hybridized carbon of an allyl carbon is the allylic carbon.

4. Each of the following allyl groups are resonance stabilized and the charge or unpaired electron is shared between the two end carbons. Resonance Forms and Delocalization

5. In resonance structures that are not equivalent the resonance structures do not contribute equally to thehybrid. Resonance Structures The positive charge is shared between the two carbons that have positive charge in the individual resonance structures.

6. In valence bond theory a p-bond that covers all threecarbon atoms is generated from the three adjacent porbitals. Valence Bond Theory and AllylCations

7. The resonance stabilized carbocation is much more stable so the rate determining ionization step is much faster. Allylic Halides and SN1 Reactions Allylic halides react much faster than tertiary alkyl halides.For example:

8. Since the positive charge is shared between two carbonsthe water attaches to both carbons to form two products: Allylic Resonance and SN1 Reactions Intermediate cation

9. Reaction Equation. Mechanism of Hydrolysis of Allylic Chlorides Step 1. Ionization. Step 2a. Addition of water to one end. Step 2b. Addition of water to the other end.

10. Mechanism of Hydrolysis of Allylic Chlorides Step 3a. Deprotonation. Step 3b. Deprotonation. The major product corresponds to the more stable resonance structure.

11. Any other allylic chloride that forms the same stabilized cation should form exactly the same mixture of products. Confirmation of Mechanism Indeed this is the case: Common intermediate cation

12. For SN2 reactions we compare reactions of a strong nucleophile with a series of primary alkyl halides including allylic halides. Allylic Halides and SN2 Reactions

13. SN2 reactions occur when allylic chlorides react with good nucleophiles. SN2 Reactions of Allylic Chlorides

14. Allylic Free-Radical Halogenation

15. The allylic radical is stabilized by electron delocalizationexpressed as resonance between Lewis structures andshould be easy to form selectively. Allylic Radical Stablization

16. Delocalization of the unpaired electron stabilizes the allylic radicals so the bond dissociation enthalpies arelower. Quantifying the Stability of Allylic Radicals The allylic radical is 55 kJ/mol more stable than propyl radical.

17. Free radical reaction of propene with chlorine at high temperatures is an industrial process. Allylic Chlorination

18. Mechanism of Allylic Chlorination Step 1: Bond dissociation. Initiation Step 2: Hydrogen atom abstraction. First propagation step. Step 3: Chlorine atom abstraction. Second propagation step. The two propagation steps repeat over and over.

19. Allylic brominations are usually carried out with N-bromo-succinimide. Allylic Halogenation

20. Allylic halogenation is a selective reaction provided that all the allylic hydrogens are equivalent and lead to equivalent resonance forms. Allylic Halogenation alkene equivalent radical resonance forms

21. If the allylic resonance forms are not equivalent then a mixture of isomeric products is formed. Unselective Allylic Halogenation The allylic radical reacts through the two resonance structures shown below.

22. Allylic Anions

23. The allylic anion is planar and stabilized by electron delocalization. In resonance terms: Allylic Anions The electrostatic potential map shows the electrondelocalization as compared to propyl anion. Red showshigh electrondensity

24. A comparison of the pKa’s of propane and propene give a measure of the relative stability of the two anions. pKa and Stability of Allylic Anions

25. Classes of Dienes:Conjugated and Otherwise

26. Dienes may be:Conjugated in which the two alkenes are joined by a C-Csingle bond; Isolated in which there is at least one sp3 carbon between the alkenes; Cummulated in which two alkenes share a carbon. Classes of Dienes

27. Alkadienes are named by replacing the -ane ending and replacing it with –adiene and adding the locant for each alkene. Naming Dienes (E)-1,3pentadiene 1,6-heptadiene 1,4-cyclohexadiene

28. The stabilities of conjugated, cummulated and isolated dienes can be estimated by comparing the heats of hydrogenation. Relative Stabilities of Dienes The conjugated diene is 15 kJ/mol more stable than the isolated diene. Isolated diene = two separate alkenes.

29. Relative Stabilities of Dienes The cummulated double bonds of allene are relatively high energy. The heat of hydrogenation of allene is 45 kJ/mol more than twice the heat of hydrogenation of propene!

30. The single bond that separates the two conjugated dienes is relatively short. Bonding in Conjugated Dienes

31. Conjugated dienes are more stable than isolated dienes because there is greater electron delocalization. With onesp3 carbon between alkenes the p orbitals are separated and cannot overlap. Bonding in Conjugated Dienes conjugated isolated

32. There is maximum orbital overlap and electron delocaliza-tion if the two dienes are coplanar. The two coplanar conformations are called s-cis and s-trans. Conformations of Conjugated Dienes

33. Bonding in Allenes

34. The central carbon which form two p bonds is sp-hybridized and the other two carbons are sp2 hybridized. Allene innon planar. Bonding in Allenes

35. In these diagrams the p-orbitals are shown in different colors to better show their mutually orthogonal spatial arrangement. Bonding in Allenes p orbitals C1C2 p orbitals C2C3 p orbitals C1C2 and C2C3

36. The nonplanarity of allenes means that 1,3-disubstituted allenes are chiral. Chirality with Allenes

37. Thermal dehydrogenation in the presence of catalysts is an industrial process. Preparation of Dienes In the lab dehydration or dehydrohalogenation (eliminationreactions) are used.

38. Rubber is a natural polymer made from isoprene. Diene Polymers The polymer consists of isoprene units connected together. Rubber has(Z)-alkenes. Gutta Percha has (E) alkenes and is more durable and was used to insulate undersea communication cables.

39. Addition of Hydrogen Halides to Conjugated Dienes

40. Addition of hydrogen halides to alkenes is a characteristic reaction of alkenes. Addition of HX to Conjugated Dienes

41. Step 1. Protonation to give an allylic cation. Mechanism of Addition of HX Step 2. Attack of chloride. In this reaction both resonance forms of the allylic cation are equivalent so only one product is formedas a mixture of enantiomers.

42. Addition of HX to conjugated dienes that form two nonequivalent intermediate allylic carbocations yield mixturesof two products. Addition of HX to Conjugated Dienes

43. The ratio of the two products is temperature dependent. Kinetic and Thermodynamic Products At -80 oC the ratio is 81:19, at 25 oC it is 44:56 and at 45 oC it is 15:85. At low temperatures the fastest formed product is preferentially formed – the kinetic product. At high temperatures the more stable product is formed –the thermodynamic product. More substituted double bond!

44. Kinetic and Thermodynamic Products thermodynamic kinetic The bromide is generated close to C-2 so addition to C-2 is faster. Kinetic product. For the ratio to change with temperature the reactions have to be reversible so the 1,2 addition product must ionize to reform the allylic cation and then form the 1,4-addition product.

45. The activation energy to form the 1,2-addition product islower so it is more readily formed. For the 1,4-addition product this is reversed. Kinetic and Thermodynamic Products

46. Halogen Addition to Dienes

47. 1,4-Addition predominates with (E) products preferentially formed. Halogen Addition to Dienes

48. The Diels Alder Reaction

49. The Diels-Alder reaction is a cycloaddition reaction of a diene and an alkene (called a dienophile). It is an example of a pericylclic reaction (cyclic transition state). The Diels-Alder Reaction No catalyst is needed.

50. Electron withdrawing groups on the alkene enhance the rate of reaction. Carbonyls (C=O) are electron withdrawing groups. Substituents and the Diels Alder Reaction Two electron withdrawing groups makes the dienophile even more reactive.